1. Field of the Invention
The invention relates to an apparatus for controlling the air-fuel ratio in a direct-injection internal combustion engine. More specifically, the invention relates to an apparatus for controlling the air-fuel ratio, which performs feedback control according to a fuel injection mode.
2. Description of the Related Art
Direct-injection internal combustion engines, which are provided with fuel injection valves (hereinafter referred to as “fuel injectors”) that inject fuel directly into the cylinders, offer improved fuel consumption and output characteristics. As a result, the use of these types of engines is expected to further increase in the future. When these types of internal combustion engines employ a single-injection mode in which a substantially large quantity of fuel is injected at once in a single injection during one cycle, some of the injected fuel tends to bounce off of the piston head and adhere to the spark plug at low temperatures which hinder the atomization of injected fuel. In particular, immediately after a cold start when the engine has not yet sufficiently warmed up, the amount of fuel injected is increased in order to facilitate combustion. This increase in the amount of injected fuel, however, in turn results in an increase in the amount of fuel adhered to the spark plug, thereby fouling the spark plug. This fouling tends to hinder the effective sparking of the spark plug and thus effective ignition of the air-fuel mixture. To avoid this problem, art is proposed in Japanese Patent Laid-Open Publication No. 2000-45840 which employs a multiple-injection mode in which some of the fuel is injected during the first half of the intake stroke and some during the second half of the intake stroke. More specifically, this art uses a multiple-injection mode in which more fuel is injected during the first half of the intake stroke than during the second half of the intake stroke, thereby reducing the amount of fuel adhered to the spark plug.
To operate the engine at stoichiometric combustion in the single-injection mode, feedback control is performed based on a value output from an air-fuel ratio sensor provided in an exhaust passage, such that the weight ratio of air being drawn into the cylinder to fuel (hereinafter referred to as “engine air-fuel ratio”) becomes the stoichiometric air-fuel ratio. By maintaining the engine air-fuel ratio at the stoichiometric air-fuel ratio and effectively purifying the exhaust gas with a fully functioning three-way catalyst, deterioration of the exhaust emissions is able to be minimized. Similarly, even when operating at stoichiometric combustion in the multiple-injection mode, it is still necessary to minimize deterioration of the exhaust emissions.
The inventors have confirmed a phenomenon in which the amount of NOx emissions increases when the air-fuel ratio in the multiple-injection mode is controlled using a feedback control method for the single-injection mode. According to the inventors, a conceivable explanation for this phenomenon is as follows. The oxygen concentration in the exhaust gas is detected by an air-fuel ratio sensor, which is typically an O2 sensor or an A/F sensor. Feedback control is then performed on the engine air-fuel ratio based on that detected value. The construction of the air-fuel ratio sensor makes it vulnerable to being adversely effected by hydrogen. When fuel is injected a plurality of times in one cycle at low temperatures, the distribution of the fuel inside the combustion chamber is uneven compared to when it is injected only once at normal temperature. As a result, good combustion conditions are unable to be maintained, which results in the exhaust gas containing a large quantity of hydrogen. This hydrogen reacts with the oxygen inside the air-fuel ratio sensor, such that the air-fuel ratio sensor obtains a detection value that indicates a richer air-fuel ratio than the actual engine air-fuel ratio when the engine is operating in the multiple-injection mode. As a result, an engine air-fuel ratio that is actually lean may be erroneously detected as being rich. It can then be presumed that, based on this erroneous detected value, feedback control would then attempt to shift the engine air-fuel ratio toward the lean side, which would result in the engine air-fuel ratio being leaner than the stoichiometric air-fuel ratio, thus increasing the amount of NOx emissions.